Various back draft dampers have been used at the outlet of exhaust fans, normally operable to exhaust air from kitchen or bathroom areas, and these back draft dampers are usually light weight flapper valves which open in response to pressure of air from an exhaust fan and close by gravity when the air pressure from the exhaust fan is shut off. The conventional back draft dampers are designed to prevent air from flowing through the area of an exhaust fan and to an area outside of a building room and also disposed and adapted to prevent air flow from the outside into the room. These back draft dampers are generally simple check valve mechanisms.